Field of the Invention
The invention relates to a random light collector device.
Description of Related Art
Physical systems dealing with random light emission sources in general use a vapor or beam of atoms or molecules generated in a vacuum, wherein at a desired position along the beam the atoms or molecules included in a small axial section of the beam are excited by a pump laser beam which intersects the beam of atoms or molecules, for example at a right angle. The laser beam has a precisely defined wavelength suitable to cause a specific fluorescence due to the excitation of the atoms or molecules from a specific ground state to a specific excited state. The fluorescent light is detected and the wavelength of the pumping laser may be adjusted in such a way that the detection signal, which is proportional to the optical power of the fluorescent light, is maximized or minimized depending on the specific application.
Physical systems using such random light emission sources are, for example, frequency standards for GPS satellites. Especially cesium beam frequency standards have met and exceeded the stability requirements necessary to achieve the timing and position accuracy of GPS systems. Also beams of any other alkali metal atoms like rubidium atoms may be used to realize frequency standards.
One of the major problems for realizing such frequency standards is the detection of the fluorescent light with a suitably high efficiency. The light source realized by the pumping beam which intersects the beam of atoms or molecules (in the following, the term “atoms” is used as a general term for atoms and molecules) can be considered as point source imaging non-directive light, i.e. the randomly emitted photons cover a 4π steradian solid angle. As the power of the fluorescent light is very low—a few photons per atom are emitted only while the atom crosses the pumping zone—the light collection efficiency has to be maximized. That is, as much photons as possible must be detected which means that every photon should be detected independently of the solid angle into which it is emitted.
One solution is to position the photodetector as closely as possible to the atom emitting source. However, this is in many cases technically impossible, especially as many photodetectors having desired properties (especially a large sensitive area and a low signal-to-noise ratio) cannot be used in a vacuum environment. Further, the creation of the atom beam and the pumping zone must be shielded against disturbing magnetic fields, which does not allow to position ferromagnetic elements or materials within this shielded zone.
By positioning the photodetector away from the emission source, the detection efficiency significantly decreases due to the reduced solid angle covered by the photodetector. Using a larger photodetector or an array of photodetectors is not an appropriate partial solution to this problem as this would lead to an increase in its noise contribution and thus limit its signal-to-noise ratio to an unacceptable level.
In order to improve the fluorescent light detection efficiency while remaining compatible with technical requirements, a closed mirror assembly with means for guiding light is known, for example from T. Bondo et al., “Time-resolved and state-selective detection of single freefalling atoms”, Mar. 9, 2006 (downloadable from: http://www.researchgate.net/publication/224045239_Time-resolved_and_state-selective_detection_of_single_freely_falling_atoms). This closed mirror assembly comprises an ellipsoid concave mirror and a spherical concave mirror. The random light source is provided in the first focal point of the ellipsoid mirror (the focal point closer to the respective mirror surface) and the second focal point of the ellipsoid mirror is essentially positioned on the surface of the spherical mirror. The focal point of the spherical mirror coincides with the first focal point of the ellipsoid mirror. The spherical mirror has a circular opening of a given diameter located at the second focal point of the ellipsoid mirror, i.e. in the intersection of the spherical mirror and the optical axis formed by the two focal points of the ellipsoid mirror. The random light source is realized by a beam of slowly moving rubidium atoms which is intersected by a pumping laser beam at the first focal point of the ellipsoid mirror. A telescope consisting of several lenses, an aperture and a filter is used to focus the fluorescent light onto the sensitive area of a photodetector. Due to the configuration of the telescope, the image of the fluorescent light in the plane of the output port is reproduced on the sensitive area of the photodetector, practically without major distortions.
This random light collector device reveals the disadvantage that, in addition to the transparent window of the vacuum chamber, each of the three lenses and the additional optical band pass filter of the telescope attenuate the light to be detected. Further, the lenses must be positioned radially with respect to the optical axis and axially with respect to each other and the photodetector, respectively.
It is thus an object of the present invention to provide a random light collector device, especially for realizing a frequency standard, which reveals an improved collection efficiency, which has a simpler design resulting in lower manufacturing costs.